Shaft drilling benefits from improved technology; new equipment and methods merge to produce deeper, wider, and straighter holes.
The evolving science of underground excavation was brought into sharper focus at the 1991 technical conference of the Institute for Shaft Drilling Technology. Approximately 120 drilling contractors, equipment vendors, and others met in Las Vegas to hear more than 30 speakers discuss topics ranging from new developments in blind-hold drilling and raise boring to status updates on major projects.
Shaft-drilling technology has achieved significant recent advances, with improvements in boring-machine cutter design, accuracy of drilled pilot holes for raise boring, and methods of shaft-wall support. Yet, with the technology largely in hand to bore holes from less than a yard to 25 ft in diameter, the industry's capabilities aren't widely recognized. "The real objective of the ISDT is to share and expand a technology that is known by relatively few people and described through a sparse number of technical papers," states Hassell E. Hunter, an institute director.
Increasing use of mechanical methods to sink shafts in varying ground conditions lends credence to ISDT's claim that shaft drilling is gaining acceptance as an alternative to conventional drill-and-blast procedures. Although record-setting projects involving large-diameter drilled shafts more than 1,000 ft deep usually grab the headlines, ISDT officials point out that there's plenty of activity in shallow-shaft work - with more than 500 shafts less than 1,000 ft deep at the U.S. nuclear test site at Yucca Flats, Nev., alone.
However, a major obstacle to acceptance of blind drilling by the mining industry has been the lack of precise hole verticality needed for hoisting operations. The big rotary table rigs capable of drilling the large-diameter holes needed by mine operators depend on a "pendulum" effect imparted by the weight of the drill bit and string to maintain hole orientation. This requires a decreased level of weight on the bit, and leads to accelerated cutter wear and less than optimum penetration rates. Improved stabilizer assemblies and Steerable bits, being developed by Wirth GmbH, Zeni Drilling and others, may solve this problem.
Various types of shaft-boring equipment have been developed, with mixed results. The Shaft Boring Machine (SBM), developed in 1984 by a joint venture involving J.S. Redpath Corp. and The Robbins Co., illustrates the uncertain demand for high-tech boring equipment.
The SMB turned the concept of tunnel boring on its head, taking the approximate configuration of a tunnel-boring machine but substituting a rotating wheel equipped with disk cutters to blind-bore large-diameter vertical holes (see accompanying figure). The wheel, pressed against the rock by hydraulic force, slews in a circular motion around the shaft bottom as it rotates. Muck is removed by a scraper and clamshell device which loads a bucket hoist.
According to Thomas M. Goodell, senior manager of engineering for Redpath Engineering Inc., estimates indicate the cost of boring a shaft with the SBM compares favorably with drill-and-blast operations to a depth of about 1,500 ft; below that, SBM has a clear cost advantage.
Despite its projected cost-competitiveness and the prospect of important operational and safety benefits compared with conventional methods, demand hasn't materialized; it now sits idle at Redpath's Gilbert, Ariz., facility.
Raise-boring and down-reaming systems have fared better, particularly in applications involving small- to intermediate-diameter shafts in soft- or medium-hard rock. Two of the more challenging projects to recently utilize these techniques were described at the ISDT conference.
Raise-boring of a 6-m-dia, 1,100-m-deep ventilation subshaft at South Africa's Kloof No. 4 gold mine during 1989-90 was a daunting task for Rucbor (Pty.) Ltd., which calls the project "the ultimate challenge." The job attained several raise-boring milestones, said Rucbor, including:
* The world's deepest raise-bored shaft;
* Penetration of the hardest rock formation yet encountered in raise-boring operations; and
* The first shaft of this size to be raise-bored from a starting point so deep (2,000 m underground, to 3,100 m).
F.C. Roos of Rucbor explained that the ventilation shaft was part of a program to extend the mine's underground workings to a depth of 3,500 m. Preliminary geological estimates, on which Rucbor's raise-boring proposal was based, indicated that the unconfined compressive strength of the rock through which the ventilation shaft was to be drilled would not exceed 300 MPa, but subsequent core drilling showed that 80% of the ground to be drilled had a compressive strength of 520-579 MPa (75,400-83955 lb/[in..sub.2]). Despite the extremely hard ground, Rucbor was able to maintain an average advance rate of 4.4 m/d, just under its proposed rate of 5 m/d.
Rucbor used a Wirth HG 330 SPII raise drill (see table), a 12-7/8-in.-dia integral-type drill string, and a Sandvik CRH-10-E reaming head fitted with 24 cutters. The pilot hole was drilled to a depth of 1,102 m within a tolerance of 1% of the linear length of the hole. In-hole surveys were performed using a Servo Datadrill.
In order to maintain an acceptable rate of advance, force on the pilot bit had to be increased from 30 mt at the outset to 40 mt at 150 m below the collar, and finally to 45 mt at a depth of 450 m. Despite the high level of bit loading at this depth, penetration dropped to 0.7 m/hr because of the rock's extreme hardness. Pilot-bit life, originally estimated at 138 m/bit, averaged 74 m for the 15 bits used to drill the hole. Much of the drilling was carried out at penetration rates of 0.70-0.8 m/hr, well below the planned rate of 1 m/h.
Reaming operations began on Sept. 20, 1989, one year after pilot drilling had commenced. Working torque on the 530-mt head assembly and drill string was initially limited to 400 kNm (295,000 ft-lb) to avoid rod overloading, but was eventually raised to 440 kNm to produce an acceptable advance rate in harder rock. The maximum thrust available at 400 kNm was 8,000 kN (1.8 X [10.sub.6]), or 12 mt/cutter. At the higher torque, cutter loading reached 20 mt/cutter.
After only 20 m of reaming, Rucbor experienced a problem when the reamer's gauge cutters intersected a previously developed muck-removal tunnel. The highly fractured ground near the tunnel caused the reamer to hammer, resulting in extensive damage to the head and cutters. The head was recovered, replaced, and reaming recommenced 17 days later, only to encounter extremely hard rock (517 MPa) which slowed penetration to 3 m/d and drastically shortened cutter life.
It became apparent that cutter loading had to be increased to extend cutter life, and the reamer itself needed strengthening to withstand shock loading from drilling in blocky, hard rock. Engineers from Rucbor, Sandvik, and Kloof identified and agreed upon the needed modifications, which took 32 days to complete. The reamer was reinstalled and subsequently maintained an advance of 4.56 m/d. But, a drill rod broke after about 400 m of reaming, causing a 36-day delay to recover and replace the reamer.
In recapping the project for the ISDT attendees, Rucbor's Roos pointed out that the 4,500 hr of actual reaming time required to complete the shaft were not out of line with the 4,200 hr estimated at the outset. Several other factors contributed to delays, including:
* Rod handling time. The reamer had to be lowered 15 times for cutter changes, nine more than originally planned, because of the adverse ground conditions.
* Excessive mucking delays. The mucking station and about 20 m of shaft had to be cleaned out whenever the reamer was lowered, amounting to approximately 40 days of unplanned mucking time.
* Delays from rod string failure, reamer modification, and reamer repairs, totaling 119 days.
SHAFT BORING AT BROKEN HILL
Rucbor also participated in a joint venture during 1990 with Thyssen Schachtbau of Germany to bore an 810-m-deep ventilation shaft at Pasminco's Broken Hill mine in New South Wales, Australia, this time using a Wirth V-Mole to down-ream a 1.8-m-dia pilot hole to 6-m-dia.
The introduction of larger diesel-powered equipment into the mine required larger volumes of air in the underground workings, and two new shafts were needed to carry downcast and upcast air. Previous experience at Broken Hill in boring pilot holes and reaming them to larger diameter had not been good, said Pasminco's N.J.H. DeBruin.
The first of the new shafts, No. 4, was originally designed to be 6 m in diameter, and since access was available at the shaft bottom, it was decided to construct the shaft by means of full-diameter raise-boring. However, due to adverse ground conditions, this approach proved unworkable and the final shaft diameter was limited to 4 m. Conventional drilling techniques were similarly unsuccessful when drilling pilot holes; the No. 4 shaft pilot deviated more than 32 m over its 900-m depth.
The second shaft's pilot hole was drilled using controlled navigational-drilling techniques that limited deviation to 650 mm maximum. The hole was then back-reamed to 1.8-m-dia in preparation for the V-Mole, which was assembled and installed between March and July 1990.
Geotechnical studies indicated that about 186 m, or 22.5% of the shaft could be left unlined. However, these stable areas were distributed over seven locations along the overall shaft length. Because of this wide dispersion, it was determined that the entire shaft would be lined with concrete, using a two-deck stage that followed the V-Mole.
Reaming of the shaft began on July 25, 1990. It soon became apparent that the dust generated by the reaming operation was contaminating air in various parts of the mine's workings, and that reaming would have to be confined to limited hours during the day until different ventilation arrangements could be made. Because of this restriction, only 113.7 m were reamed between the starting date and August 29. The average rate of advance was 3.8 m/d.
Once the new ventilation plan was in place, the average daily advance rose to 7.5 m/d down to a depth of 468 m, where problems with pilot hole deviation and increasingly hard rock caused cutter damage and slowed the advance to 2.7 m/d. Because of the compressive strength (351 MPa) of amphibolite encountered at 506 m, it was decided to strip 0.4 m from the pilot hole's diameter along 39 m of its length by means of drilling and blasting. It took 29 working days to drill through the section of the shaft from 468 to 545 m.
The remainder of the job proved uneventful, and reaming was completed on January 15, 1991. Over the course of the project, the maximum reaming rate achieved was 12.10 m/d, and advances of more than 10 m/d were frequent, according to DeBruin (see table). He and co-author C. Cetindis of Thyssen Schachtbau cite the project results as convincing evidence that mechanical cutting of hard rock can be done reliably, safely, and where bottom access is available for muck removal, quickly as well.
DEVELOPMENTS IN GROUTING, LINING
As existing mines expand their underground workings and the search for new orebodies goes deeper, methods that can speed the completion of underground space or expedite repairs to existing structures will assume greater importance. Although techniques such as grout injection for water control and sprayed-concrete shaft linings have been field proven for many years, new wrinkles continue to emerge in their development and application. The Pasminco ventilation shaft project, for example, benefited from a relatively recent ground-support innovation.
Prior to the Broken Hill mine ventilation-shaft reaming operation, geotechnical data indicated that waterflow into the 1.8-m-dia pilot hole could reach 1,000 [m.sub.3]/d, a volume that would strain the mine's pumping and ventilation systems. Grouting was carried out to a depth of 120 m, using 126 mt of cement injected into boreholes arranged in a 9-m radius around the shaft.
However, when the pilot hole was completed, water inflow was measured at 300 1/min at the hole bottom; obviously, enlarging the pilot to its final diamter would produce a much higher flow. Additional grouting, using SCEM 66, reduced the inflow from a maximum of 850 1/min during reaming to 15 1/min at project completion.
SCEM 66 is a patented emulsion grout developed to provide water-leakage control in underground workings and surface structures. The grout, produced under license, is a blend of latex emulsions and additives that promote flow and adhesion. The material is stable until activated by another chemical, at which time it becomes unstable and coagulates to form a jelly-like plug of matted rubber.
In low water-flow situations, the grout can be activated solely by agitation as it is injected into a small orifice or narrow fissure. The high shear action encountered during injection causes the dispersed rubber particles to flocculate and adhere to the walls of the host cavity. The flocculated rubber builds up on the walls until the cavity is filled. Addition of an inhibitor can delay the rate of coagulation.
The grout, in use since the late 1970s, was developed by J. Grobler, a South African inventor. It has reportedly been used successfully at mines in England and South Africa to solve above-and below-ground water-inflow problems.
Caledonian Mining Ltd., a British company, has devised a method for quickly and economically spraying cementitious lining material remotely in newly finished shafts. According to B. Mason and M. Bishop of Caledonian, a thin coat of thixotropic cement, rapidly applied by a spinning disk lowered into the shaft, seals the friable strata and prevents early deterioration of the host rock.
The technique draws upon 25 years of company experience in using fibrous sprayed concrete in shafts and illustrates how a thin, final liner with good flexural properties can retain rock and maintain long-term shaft-wall integrity. Since the unlined shaft must be temporarily self-supporting, this method is suited for raise-bored openings.
The process consists simply of pumping a properly formulated grout onto a horizontal spinning disk equipped with guide vanes. The disk is attached to an eight-legged support structure at the surface that keeps the disk centered in the shaft bore. The grout is propelled from the disk onto the shaft wall, where its high momentum drives the material into crevices and forces the heavier cement particles closer to the rock/ grout interface for a denser mix.
The quick-drying characteristics of the grout allow operators to apply several thin coats rather than one thick coat that may require extensive drying time, while use of a thin, free-flowing mixture avoids pumping problems encountered with thicker grouts. Production rates of up to 1,500 m/d have been achieved, said the British engineers, by applying ten complete coats of 1-mm-thick grout to a 150-m-deep shaft during a normal work day.
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|Author:||Carter, Russell A.|
|Publication:||E&MJ - Engineering & Mining Journal|
|Date:||Dec 1, 1991|
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